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When you use keys you buy a coffer. That coffer gives you a random shoulder -- either between all of the ones that NPC offers, or between two different ones if you buy a more specific coffer.
This is not tied to which dungeon you have previously run.
Ah I thought if you bought the 5key coffer you got the specific one for the dungeon listed on the coffer.
You're right that the 5 key one is more specific, but it draws from two dungeons, not one. In your case Darkshade Caverns 1 and 2. So you get either Sentinel or Engine Guardian!